console.log('2s, index4.css加载完毕')
let s = 0;var start = Date.now();
for(let i = 0; i < 100 * 1000 * 1000;i++){
    s+=i;
}
// 1亿: time 1115   1千万：time 175
console.log('3s js执行完成, 然后渲染')